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CREDITS: (TOP) BESWICK ET AL., J. AM. CHEM. SOC. 10.1021/JA026079K (2002); (BOTTOM, COUNTERCLOCKWISE) (FIRST TWO IMAGES) DOUG WECHSLER/VIREO; SAM FRIED/VIREO; T.J. ULRICH/VIREO
H I G H L I G H T S O F T H E R E C E N T L I T E R AT U R E
EDITORS’ CHOICE
edited by Stella Hurtley
IMMUNOLOGY
Deactivation by
Degrees
The cell-surface tyrosine phosphatase, CD45, provides critical
regulation of lymphocyte activation. By dephosphorylating
inhibitory tyrosine residues on
protein tyrosine kinases (PTKs),
CD45 facilitates signaling
through the T cell receptor. This
phosphatase activity is constitutive in the CD45 monomer
but is down-regulated through
dimerization.
CD45 is expressed as distinct
isoforms, generated through alternative exon splicing of the
extracellular portion of the
molecule. Xu and Weiss now
show that these isoforms are
the key to how dimer formation
is regulated. The smallest isoform (CD45RO) dimerizes more
readily than its longer counterparts, correlating with reduced
levels of T cell activation and reduced levels of posttranslational
modification by O-linked glycosylation and sialylation. Thus,
longer isoforms, which are more
prevalent on resting
T cells, appear to
be preferentially maintained as
active monomers because increased sugars would impede
dimer formation. Production of
smaller isoforms after T cell
activation would thus promote
dimerization, reducing CD45
phosphatase activity and
dampening down T cell
responses. — SJS
Nature Immunol. 3, 764 (2002).
GEOLOGY
Pangea Weather
Report
The final assembly of the supercontinent Pangea, toward the
end of the Paleozoic Era, built up
an enormous, high-standing land
mass that straddled Earth’s
equator. Those were excellent
conditions for the development
of a monsoon in the supercontinent’s equatorial regions, and
paleoclimate models have suggested that a transition from
zonal to monsoonal circulation
patterns had probably taken
place by the early Permian Period, approximately 280 million
years ago.
Now Soreghan et al. present
some persuasive geological
evidence for such a transition.
They employed an ingenious
tracer: the isotopic ages of detrital zircons in Upper Paleozoic
loessites (rocks formed from
deposits of windblown silt) in
the southwestern United
States, which marks the late
Paleozoic location of western
equatorial Pangea. Tying the
isotopic ages to regional source
rocks exposed during the late
Paleozoic, they inferred predominant northeasterly winds
during the middle Pennsylvanian Period (290 to 300 million
years ago), but a combination
of westerly and easterly wind
patterns during the early Permian, a pattern consistent with a
shift from zonal circulation to a
seasonal monsoon. — SW
Geology 30, 695 (2002).
C H E M I S T RY
Charging Ahead
Dithiolenes, which contain the
–S–C=C–S– group, can form
complexes with transition metals that display unusual magnetic or redox properties and
can provide insights into the reactivity of metal-sulfur clusters
E C O L O G Y / E VO L U T I O N
Radiation Revised
Darwin’s
finches.
With their diverse bill shapes and ecological habits, Darwin’s finches—13 species from the Galápagos Islands
and 1 from Cocos Island—serve as a classic example of
adaptive radiation. Understanding the evolution of the
group and its morphologies has been handicapped by a lack of
agreement about these finches’ closest living relatives.
Burns et al. analyzed cytochrome b mitochondrial DNA sequences from the
finches, sister taxa, and potential outgroups within the finch-tanager tribe.
Darwin’s finches form a clade within a larger, well-supported monophyletic
group of species that build domed nests. Most of these close relatives are
endemic to Caribbean islands and not found in South America. The
relatives have a diversity of bill morphologies and feeding behaviors similar to those of Darwin’s finches, and this diversity
also evolved rapidly. The parallel courses may reflect strong
selection as birds colonized islands with vacant niches, a developmental-genetic architecture inherited from the common ancestor, or both. In any case, it appears that the key
change in the clade’s history occurred before its arrival in the
Galápagos. — ShJS
Evolution 56, 1240 (2002).
in enzymes. Normally, if the
complex has only dithiolene ligands, all of the potential sites
A view of the cube shape showing Pd (blue) and S (red) atoms.
for ligand binding are occupied:
The compound is coordinately
saturated.
Beswick et al. now report on
the synthesis and characterization of an air-stable palladiumdithiolene complex in which six
Pd-S2C2(COOCH3)2 units adopt
a slightly distorted cube-octahedron structure. The Pd atoms
occupy the center of the cube’s
faces and the S atoms occupy
the midpoints of the edges,
leaving the center of the cube
and the opposite site on each
Pd atom unoccupied. Cyclic
voltammetry studies show that
the neutral complex can be reduced reversibly to form the 4–
anion. This stable unsaturated
cluster should provide a useful
starting point for the synthesis
of related compounds. — PDS
J.Am.Chem.Soc. 10.1021/ja026079k (2002).
A S T RO P H YS I C S
Planets in the Mist
T Tauri stars are considered active adolescents in stellar
terms (less than 10 million
years old). When they grow up,
they will become solar-type
stars. Classical T Tauri stars are
less than 1 million years old
and possess a circumstellar
disk from which gaseous giant
planets may form. It has been
thought that it would take
more than 1 million years to
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SCIENCE
VOL 297
16 AUGUST 2002
1095